Photomasks are used in the patterning of integrated circuit devices, and a conventional photomask may include microscopic images on a transparent substrate. As shown in FIG. 9, a photomask 901 may include a transparent substrate 903 (such as a glass and/or quartz substrate) with a microscopic image provided in a patterned layer 905 (such as a patterned layer of chrome) on a surface of the transparent substrate 903. The microscopic image may be transferred from the photomask to a photosensitive layer (such as photoresist) on a integrated circuit wafer using photolithographic techniques wherein patterning radiation (such as light) is transmitted through the photomask onto the photosensitive layer. Accordingly, portions of the photosensitive layer corresponding to openings in the patterned layer of chrome 905 are selectively exposed to the patterning radiation, while portions of the photosensitive layer corresponding to chrome portions of the patterned layer are masked from the patterning radiation. After exposure to the patterning radiation through the photomask, the photosensitive layer can be developed to form the desired pattern in the photosensitive layer. The photosensitive layer having the desired pattern can then be used as an etching mask for a layer of the integrated circuit device.
A critical dimension of an integrated circuit pattern may be defined as a width of a line or space that has been identified as being critical to the device being fabricated. Portions of the pattern on the transparent substrate may include a same critical dimension across different portions of the photomask, and the critical dimension is ideally reproduced uniformly in the photosensitive layer across different portions of the integrated circuit device. A uniformity of a critical dimension formed at different portions of an integrated circuit device, formed at different integrated circuit devices on a same semiconductor wafer, and formed on different semiconductor wafers may vary, however, even when using a same photomask.
According to current photolithographic technologies, a photomask may include a pattern for a layer of a single integrated circuit device or a small number of adjacent integrated circuit devices, but not for all integrated circuit devices on a wafer. Accordingly, the photomask may need to be “scanned” or “stepped” to separately expose different parts of a same photosensitive layer on a same wafer. For a photomask including a pattern for a layer of a single integrated circuit device, the photomask may need to be sequentially aligned to each integrated circuit device on the wafer and a separate dose of patterning radiation may need to be provided for each integrated circuit device.
Non-uniformity of a critical dimension across a semiconductor wafer and/or from wafer to wafer using a same mask may be a result, for example, of variations in coating the photosensitive layer, variations in exposing the photosensitive layer to the patterning radiation, variations in developing the photosensitive layer after exposure, variations in baking the photosensitive layer, and/or variations in etching a layer on the wafer using the patterned photosensitive layer as a mask. Non-uniformities of a critical dimension across a same integrated circuit device may be a result, for example, of differences in intensities of patterning radiation reaching the photosensitive layer on the integrated circuit device.
Even if critical dimensions of the patterned layer of the photomask are provided without error, characteristics of the exposure apparatus and/or of the photomask may introduce non-uniformities in critical dimensions formed on a photosensitive layer. For example, a critical dimension of a pattern formed in a photosensitive layer at a center of an integrated circuit device may generally be larger than a critical dimension of a pattern formed in the same photosensitive layer at an edge of the same integrated circuit device. This effect may be due to diffraction of the patterning radiation passing through the patterned layer of the photomask.
As shown in FIG. 10, patterning radiation 1001 may be provided to a backside of the transparent substrate 1005 opposite the patterned layer 1007. Moreover, the patterning radiation 1001 may be provided with a relatively uniform distribution of illumination intensity as illustrated by solid line 1021. The patterning radiation may also propagate through the transparent substrate 1005 with a relatively uniform distribution of illumination intensity as illustrated by solid line 1023. A uniformity of illumination intensity passing through openings in the patterned layer 1007, however, may vary across the substrate as illustrated by the dotted line 1025. The uniformity of illumination intensity passing through the openings in the patterned layer 1007 may vary, for example, because diffraction of light passing through openings in the patterned layer 1007 may be stronger at the center of the photomask 1003 than at edges of the photomask 1003. A resulting distribution of critical dimensions across a device being patterned is illustrated by the line 1027.